The present invention relates to suppression of corrosion that occurs in water tubes or the like of boilers. More specifically, the invention relates to a boiler operating method in which the concentration of sulfate ions SO42xe2x88x92 in the boiler water is regulated. The invention also relates to a method for injecting a sulfite base oxygen scavenger which is added to water supplied to the boiler.
Generally, during the operation of a boiler, the pH value of the water within the water tubes, or of the boiler water, increases with generation of hydroxide ions due to thermal decomposition of bicarbonate ions contained in the feed water and with concentration of the boiler water due to evaporation. In this connection, in once-through boilers, which involve no circulation of water in the boiler body, in the degree of concentration of boiler water between upper part and lower part of the water tubes differs. As a result, the pH of boiler water increases considerably in the upper part of the water tubes, but does not increase so much in the lower part of the water tubes. This results in a water quality of nearly the same pH value as the feed water. Indeed carbon steel is widely used to make the water tubes in boilers, but carbon steel is prone to corrosion in neutral water containing oxygen. Thus, the water management for boilers involves control processes to remove dissolved oxygen in the feed water and raise the pH of the standards for boiler water. According to the water quality management in JIS (Japanese Industrial Standards), the pH of boiler water should be controlled to be around 11 to 11.8. If this level is regarded as a proper pH level, then pH in lower part of the water tubes is lower than the proper value. Accordingly, the once-through boiler is prone to corrosion in lower part of the water tubes. To prevent this corrosion, it is conventional to add agents to the feed water for the boiler. These agents are classified roughly into two kinds depending on components contained in the agents. One is called filming type agents, which are agents that form an anticorrosion film on the surface of the water-tube inner wall. The other is called oxygen scavenging type agents, which remove dissolved oxygen in the feed water, (a cause of corrosion), to prevent corrosion.
In past literature concerning water management of boilers, dissolved oxygen, low pH (operation of boilers at pH levels lower than proper ) and harmful ions (e.g., chloride ions clxe2x88x92, sulfate ions SO42xe2x88x92) have been mentioned as principal causes of corrosion. However, the effect of these factors on corrosion was examined no more than only qualitatively, the case being unclear. It was also unclear whether the filming type agent or the oxygen scavenging type agent, is more effective to prevent of corrosion.
Thus, the inventors have experimented to find ways to effectively reduce corrosion of boilers. As a result, it was found that the anticorrosion effect of filming type agents is, in general, largely affected by pH, and sufficient anticorrosion effect could not be expected in a low pH region, i.e., pH=7-9. That is, the anticorrosion effect is insufficient in lower part of the water tubes in the once-through boiler. On the other hand, as a result of making similar corrosion experiments with the oxygen scavenging type agents, such as ascorbic acid, erythorbic acid, MEKO (methyl ethyl ketoxime), hydrazine and sulfite as an oxygen scavenger, it was found that anticorrosion properties do not depend so much on pH (unlike the filming type agents). The oxygen scavenging type agents exhibit a superior anticorrosion effect even with water quality in lower part of the water tubes in the once-through boiler (in a low pH region). Among these oxygen scavengers, sulfite proved to have the highest anticorrosion effect. However sulfite also proved to have a potential defect in that sulfate ions SO42xe2x88x92 are generated after sulfite ions SO32xe2x88x92 have reacted with dissolved oxygen in the water. Sulfate ions SO42xe2x88x92 destroy the film anticorrosion film generated on the surface of carbon steel in the water, accelerating corrosion. As the boiler water is concentrated with evaporation, the sulfate ions S42xe2x88x92 are also concentrated so that their increased concentration causes a considerable increase in corrosion. The present invention, having been accomplished in view of these and other problems, has an object of effectively suppressing corrosion by using a sulfite base oxygen scavenger.
The present invention has been accomplished as a result of performing many studies and experiments in view of the foregoing problems. A first aspect of the invention includes regulating a concentration of sulfate ions S42xe2x88x92 in boiler water to 500 ppm or less. Also, a second aspect of the invention includes setting a concentration of residual dissolved oxygen in boiler feed water to 2-4 ppm. Further, a third aspect of the invention provides a sulfite-based oxygen scavenger composition comprising: 3-30 weight % of at least one selected from a group consisting of Na2SO3, K2SO3, NaHSO3 and KHSO3; 1-20 weight % of NaOH or KOH; 0.2-2 weight % of potassium sorbate; and 5-500 ppm of CoSO4.
By keeping the concentration of sulfate ions SO42xe2x88x92 in boiler water to 500 ppm or less, corrosion due to sulfate ions SO42xe2x88x92 is suppressed. This corrosion suppression effect becomes even more marked when the concentration of sulfate ions SO42xe2x88x92 is kept to 300 ppm or less.
According to published literature, even with a large amount of sulfate ions SO42xe2x88x92, corrosion will not occur, given enough oxygen scavenging. Among corrosion data of the literature, the presence or absence of oxygen is not specifically described in terms of its concentration, where the xe2x80x9cpresence of oxygenxe2x80x9d is unclear as to what level of ppm it is. However, the inventors have found that the quantity of corrosion is smaller when xe2x80x9cthe amount of sulfate ions SO42xe2x88x92 is small, even with oxygen left more or lessxe2x80x9d, than when xe2x80x9ca large amount of sulfite is injected in an attempt to fully remove oxygen with the result that more sulfate ions S42xe2x88x92 are generatedxe2x80x9d. More specifically, if the concentration of dissolved oxygen in the feed water after the injection of a sulfite-based oxygen scavenger is within a range of 2-4 ppm, corrosion is suppressed. Normally, the sulfite base oxygen scavenger is added in an amount over the chemical equivalent of dissolved oxygen concentration of the water to which the oxygen scavenger is to be added. However, in the present invention, the oxygen scavenger is added in an amount under the chemical equivalent so that corrosion is prevented without increasing the amount of sulfate ions SO42xe2x88x92 so much, with 2-4 ppm residual dissolved oxygen left.
In the present invention, examples of the sulfite base oxygen scavenger include sodium sulfite Na2SO3, potassium sulfite K2SO3, sodium hydrogensulfite Na2HSO3, potassium hydrogensulfite KHSO3 and the like. At least one of the forgoing sulfite base oxygen scavengers are used, but may be used in a combination of two or more, depending on the embodiment. As to the agents used for boilers, for further enhancement of anticorrosion effect, sodium hydroxide NaOH, potassium hydroxide KOH or the like is added as a pH regulator, and polyacrylate, polymaleate or the like is added as a scale dispersing agent, as required. It is noted that potassium sorbate acts as a stabilizer for the oxygen scavenger during the storage of agents, while cobalt sulfate acts as a reaction catalyst when the agents are used.